Polymeric materials that have the ability to swell may be useful in a variety of applications. The mechanism of swelling depends on the occurrence of molecular-level processes that are dictated by physical and chemical properties of the materials (Escobedo et al., Phys. Report 318:85 (1999)).
Sol-gel synthesis is a relatively simple and versatile method to prepare organic/inorganic polymers with diverse morphologies and chemical compositions (Wright, Sol-Gel Materials: Chemistry and Applications, Gordon and Breach Science Publ., Amsterdam (2001)). In contrast to hydrophilic polymers prepared from biomaterials (Khalid et al., Eur, J. Pharm. Sci. 15:425 (2002); Elvira et al., Biomaterials 1955 (2002)), dried sol-gels (or zerogels) composed of polymerized alkoxysilanes generate a SiO2 matrix that is inelastic and not prone to extensive swelling in solvents when fully dried. The lack of swelling is likely due to considerable condensation reactions that occur during the gelation and drying processes and crosslink the polymeric sol-gel material (Brinker et al., Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing, Academic Press (San Diego), Chapter 9 (1990)). Due to the inherent chemical structure of sol-gel derived materials, there are few reports of swellable silica solids prepared via sol-gel methods. Some sol-gel compositions have been formulated using amine-bridged silane precursors that swell in response to changes in pH (Rao et al., J. Sol-Gel Sci. and Tech. 26:553 (2003)) or temperature (Rao et al., Adv. Mater. 13:274 (2001); Rao et al., Adv. Mater. 14:443 (2002)) to generate “smart” materials. These amine-bridged sol-gels are only capable of very limited swelling.